Magnetic brush development apparatus for applying developer material to a latent image in an electrostatographic reproduction apparatus are well known in the art. Such apparatus may include a housing having a sump portion which contains a supply of developer material. When the developer material comprises a mixture of carrier particles and smaller pigmented marking particles, the material in the sump is agitated to triboelectrically charge the material prior to delivering it to a magnetic brush where it can be brought into association with, and electrostatically transferred to, an electrostatic latent image to develop such image.
Copending, commonly assigned U.S. patent application Ser. No. 597,3Z3, filed Apr. 6, 1984, now U.S. Pat. No. 4,887,132, issued on Dec. 12, 1989 in the names of Joseph et al, now U.S. Pat. No. 4,887,132, issued on Dec. 12, 1989 and U.S. Pat. No. 4,671,207, issued June 9, 1987, in the name of Hilbert disclose magnetic brush development apparatus particularly suitable for use with developer material having pigmented marking particles and permanent magnetic carrier particles, such as disclosed in U.S. Pat. No. 4,546,060, issued Oct. 8, 1985, in the names of Miskinis et al. More particularly, the disclosed apparatus include a ribbon blender that is used for agitating (mixing), feeding and triboelectrically charging such material in the sump portion of a magnetic brush development apparatus, and a feed mechanism that delivers material from the sump portion to a magnetic brush. The ribbon blender is constructed to provide uniform flow distribution of developer material across the length of the apparatus so that a sufficient supply of material is delivered to the magnetic brush over its full extent to develop the entire latent electrostatic image. It has been found that under certain conditions developer material tends to pump toward one end of the apparatus. Accordingly, insufficient material is delivered to areas of the magnetic brush to satisfactorily develop the entire latent electrostatic image creating copy defects such as voids or underdeveloped regions. Such conditions causing developer material pumping result from a variety of diverse parameters which include for example sump wall smoothness and developer material charge-to-mass ratio and toner concentration.
A type of ribbon blender has been described in commonly assigned U.S. Pat. Nos. 4,707,107, which issued to B. Joseph on Nov. 17, 1987, and 4,634,286, which issued to S. Pike on Jan. 6, 1987. This type ribbon blender consists of two concentric helical ribbons; the inner ribbon typically having the reverse pitch of the outer ribbon thereby inducing developer flow patterns conducive to mixing and tribocharging. The ribbons are normally attached to a common shaft and are thus rotated in the same direction.
Many variations of the ribbon configuration are possible. One configuration has the outer ribbon designed to transport material laterally toward the center of the station, and the inner ribbon configured to transport material laterally from the center toward the outside. Thus the inner and outer ribbons have reversed pitches on each half of the blender. If the direction of rotation is reversed, then the flow patterns also reverse. This configuration shows less sensitivity to flow imbalance since the blender is designed to slightly favor flow toward the center. Flow over the toning roller tends to reduce any center imbalance. The drawback of this configuration is that developer material must be removed from the center of the station when replacing developer. This necessitates removing developer stations from the machine when dumping or including a tray in the machine under the stations to catch developer as it's being dumped.
Another configuration of this type blender has the outer ribbon and inner ribbon of the same pitch the entire length of the blender. The inner and outer ribbon pitch are opposite each other. With this configuration developer material is transported laterally to one end of the station by the outer ribbon and back to the other end by the inner ribbon. The advantage of this configuration is that developer material can be dumped out the front of the station without removing the station from the machine if the outer ribbon is rotated to transport material to the front of the station. This configuration also provides a slight mixing advantage over a center balanced station since toner is brought into the station via the center ribbon the entire length of the station and must travel through the developer before reaching the toning roller. A drawback of this blender configuration is that flow balance sensitivity to material properties and hardware tolerances is increased. As a result, tighter manufacturing tolerances are required to achieve acceptable performance. If flow is not balanced, that is, if the outer ribbon doesn't transport the same amount of material at the same rate as the inner ribbon, then material will pile up at one end of the station resulting in nonuniform flow on the toning roller.